I can't use Kha's clever audio Asset handling because, in my case, the files are loaded from the server at runtime (music player-ish).

When experimenting, I notice that kha.Sound.uncompress() works for decmpressing Ogg files, but that it is in reality unusable: more than 1 second calculations for 1 second of music when running on my two year old Nokia 5 (about 25 times slower than when running the same example in Html5 desktop browser).

So, is there a way to hook into the native audio decoders to ease this burden? Preferrably the same format (mp3?) on both iOS and Android. (As far as I know, iOS doesn't have a built in decoder.)

Hey, just noticed that Sound.uncompress also calls into the native ogg decoder already. Native meaning the original C code of stb_vorbis, not using some actual hardware decoding. Nontheless your numbers seem very bad considering the current situation. Is your Kha up to date?

Nope, the vscode extension installs a pretty old version because it is itself pretty old because some Robert guy hasn't updated it in a while. Clone it from GitHub (via git clone --recursive), point the khaPath option in vscode to it and your ogg problem should be solved.
Haven't set up the StreamChannel to work like that but doesn't sound like a difficult modification so maybe just give that a try.